Beta-lactamases (β-lactamases) are enzymes (EC 3.5.2.6) produced by bacteria (also known as penicillinase) that provide multi-resistance to β-lactam antibiotics such as penicillins, cephamycins, and carbapenems (ertapenem), although carbapenems are relatively resistant to beta-lactamase. Beta-lactamase provides antibiotic resistance by breaking the antibiotics' structure. These antibiotics all have a common element in their molecular structure: a four-atom ring known as a β-lactam. Through hydrolysis, the lactamase enzyme breaks the β-lactam ring open, deactivating the molecule's antibacterial properties.

Beta-lactam antibiotics are typically used to treat a broad spectrum of Gram-positive and Gram-negative bacteria.

Beta-lactamases produced by Gram-negative organisms are usually secreted, especially when antibiotics are present in the environment.

The structure of a Streptomyces β-lactamase is given by 1BSG.

Penicillinase is a specific type of β-lactamase, showing specificity for penicillins, again by hydrolysing the β-lactam ring. Molecular weights of the various penicillinases tend to cluster near 50 kiloDaltons.

Penicillinase was the first β-lactamase to be identified: It was first isolated by Abraham and Chain in 1940 from Gram-negative E. coli even before penicillin entered clinical use, but penicillinase production quickly spread to bacteria that previously did not produce it or produced it only rarely. Penicillinase-resistant beta-lactams such as methicillin were developed, but there is now widespread resistance to even these.

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